average atomic mass & % abundance
DESCRIPTION
Average Atomic Mass & % Abundance. Average Atomic Mass. The weighted average of the atomic masses of the naturally occurring isotopes of an element Most elements occur naturally as mixtures of isotopes. Average Atomic Mass. - PowerPoint PPT PresentationTRANSCRIPT
Average Atomic Mass & % Abundance
Average Atomic Mass
The weighted average of the atomic masses of the naturally occurring isotopes of an element
Most elements occur naturally as mixtures of isotopes
Average Atomic Mass
Dependent upon both mass and the relative abundance of each of the elements isotopes
Example
Naturally occurring copper exists with the following abundances:
69.17% is Cu-63 w/ atomic mass 62.93 amu
30.83% is Cu-65 w/ atomic mass 64.93
(.6917)x(62.93) + (.3083)x(64.93)= 63.55 amu
Problem 1
3 Isotopes of Ar occur in nature
0.337% as Ar-36, 35.97 amu 0.063% Ar-38, 37.96 amu 99.6% Ar-40, 39.96 amu
Calculate the Average Atomic Mass
Answer Check
(.00337)x(35.97) + (.00063)x(37.96) + (.996)x(39.96)= 39.95amu
Problem 2
2 Naturally occurring Isotopes of Boron occur with the following abundances:
80.20% B-11, 11.01 amu 19.80% B-10, 10.81 amu
What is the Average Atomic Mass
Answer Check
(.8020)x(11.01) + (.1980)x(10.81) = 10.97 amu
Calculating & Abundance
Chlorine has two isotopes: chlorine-35 (mass 34.97 amu) and chlorine-37 (mass 36.97 amu).
What is the percent abundance of these two isotopes if chlorine's atomic mass is 35.453?
Answer Check Part 1
if 2 isotopes, then the total is 100%. assume one is x% (x), the other is automatically 100-x%, (1-x)
x(34.97) + (1-x)(36.97) = 35.453
Answer Check Part 2
x(34.97) + (1-x)(36.97)=35.453 Solve for x 34.97x+36.97-36.97x=35.453 -2x+36.97=35.453 -2x=-1.517 x=.7585 1-x=.2415
Answer Check Part 3
Therefore Cl-35 has a % abundance of 75.85% and Cl-37 has a % abundance of 24.15%
Problem 1
The two naturally occurring isotopes of nitrogen are nitrogen-14, with an atomic mass of 14.003074 amu, and nitrogen-15, with an atomic mass of 15.000108 amu. What are the percent natural abundances of these isotopes?
The atomic mass of nitrogen is 14.00674amu
Answer Check
The atomic mass of nitrogen is 14.00674amu
14.00674 = p(14.003074) + (1 -p)(15.000108)14.00674 = 14.003074p + 15.000108 - 15.000108p-0.997034p = -0.993368
p = 0.9963 = 99.63% (N14)1 - p = 0.0037 = 0.37% (N15)